A young John F Kennedy once pondered over Adolf Hitler’s death, wondering whether the Nazi leader had really died or was still alive, according to a diary he kept while travelling through Europe in 1945.

The 28-year-old future president kept a 61-page diary of his experiences, which will be up for sale at RR Auction in April. Kennedy passed the diary to Deidre Henderson, who was his research assistant at the time he was a Senator.

The diary reveals that during his time in Berlin, Kennedy wrote about visiting Hitler’s bunker only months after Germany surrendered in the Second World War.

After visiting Hitler’s mountaintop retreat, The Eagle’s Nest, the young Kennedy reflected on Hitler’s legacy in a manner that could be mistaken for admiration.

"You can easily understand how that within a few years Hitler will emerge from the hatred that surrounds him now as one of the most significant figures who ever lived," he wrote. “He had boundless ambition for his country which rendered him a menace to the peace of the world, but he had a mystery about him in the way he lived and in the manner of his death that will live and grow after him,” the future US president said. “He had in him the stuff of which legends are made.”

RR Auction’s Bobby Livingston is certain Kennedy’s legend reference was not meant as admiration for the dictator, however. “He said that in reference to the mystery surrounding him and not the evil he represented,” he said.

“This exceptional diary sheds light on a side of John F. Kennedy seldom explored and confirms America's enduring sense that he was one of the most qualified, intelligent and insightful commanders in chief in American history,” Livingston said.

Kennedy served as the 35th US President from January 1961 to November 22, 1963, the day he was assassinated in Dallas, Texas.

The diary, which is up for auction at RR Auction in Boston on April 26, is believed to be the only one he kept.

The diary also showed Kennedy expressed doubts about the role of the United Nations, writing, “In practice, I doubt that it will prove effective in the sense of its elaborate mechanics being frequently employed or vitally decisive in deterring war or peace.”